The present invention relates generally to slippage control devices for use in motor vehicles, and more particularly to a slippage control apparatus for controlling an excessive slippage of a driven wheel of the motor vehicle developed in response to start and acceleration of the vehicle.
Various types of slippage control apparatus have been known, one approach being disclosed in Japanese Patent provisional Publication No. 62-121839 in which the opening degree of a throttle valve is controlled so as to keep the slip ratio S in a predetermined range, the slip ratio being determined on the basis of a vehicle speed Vb and a driven-wheel speed Vd in accordance with an equation S=(Vd -Vb)/Vd. Another known technique involves cutting off supply of fuel into an internal combustion engine in response to occurrence of a slippage of a driven wheel of the vehicle or controlling the output torque of an engine by retarding the ignition timing of the engine.
Generally, the control of the torque of the engine, i.e., the opening degree of the throttle valve, in occurrence of wheel-slippage is based upon the frictional coefficient between the vehicle-running road surface and a tire of the driven wheel which depends on the road surface states and the types of the tire. An appropriate torque supply to the driven wheel can first be achieved under the condition of stability of the feedback control. The vehicle state during no supply of the appropriate torque thereto greatly depends upon the initial value of the operating amount of the throttle valve at the time of a start of the feedback control. That is, when the initial value is small, the drive feeling is insufficient because of too small torque and low acceleration during no supply of the appropriate torque. On the other hand, if the initial value is great, the slippage is not settled and the throttle valve results in hunting so as to frequently change the speed of the drive wheel, thereby similarly causing extreme deterioration of the drive feeling.
Further, similarly in the case of cutting off the fuel supply on the occurrence of the slippage or retarding the ignition timing, there is offered a problem that sufficient control of the slippage may be difficult or the acceleration performance can extremely drops because of fixing the cutting time of the fuel supply and the retardation of the ignition timing.